docs: def parent_images(self):
          """
          :return: list of parent images -- one image per each stage's FROM instruction
          """
          parents = []
          for instr in self.structure:
              if instr['instruction'] != 'FROM':
                  continue
              image, _ = image_from(instr['value'])
              if image is not None:
                  parents.append(image)
          return parents